This invention relates generally to improvements in photographic lenses of the type commonly referred to as a zoom lens wherein the lens is adjustable to select the magnification of the image being photographed. More particularly, this invention relates to an improved locking system for releasably locking the zoom lens in a selected magnification setting.
Zoom lenses in general are known in the art and typically comprise a plurality of optical elements mounted within a lens housing which is in turn adapted for mounting upon a camera. The optical elements of the zoom lens cooperate to provide a so-called telephoto lens capable of producing a relatively large image of a distant object. Importantly, the various optical elements are mounted on different portions of the lens housing which are mechanically movable relative to one another to permit controlled adjustment of the image focus and the image magnification.
More specifically, in a typical zoom lens, the forwardmost optical element comprises a focusing element mounted for precisely controlled axial displacement in response to rotation of a revolving housing portion to adjust the image focus. At least one optical element positioned between the focusing element and the camera constitutes a zoom element movable in response to axial sliding motion of another portion of the lens housing. Both of these movements are commonly controlled by an operating ring carried about the lens housing for appropriate rotational movement to select image focus and axial sliding movement to select image magnification.
A significant problem encountered with prior art zoom lenses is that the operating ring tends to creep axially during use or handling of the camera, resulting in an inadvertent and undesired alteration of the magnification setting. For example, the weight of the zoom optical element is frequently sufficient to cause the operating ring to creep axially under the influence of gravity whenever the zoom lens is oriented angularly with respect to the horizontal plane. Moreover, the operating ring is easily displaced from its desired position of axial adjustment by bumping or by centrifugal forces as the lens is moved or carried about. The photographer must therefore hold the operating ring manually and/or repeatedly recheck and reset the magnification setting during use of the zoom lens.
One locking system for releasably locking the operating ring in a selected position of axial adjustment is disclosed in commonly assigned, copending application Ser. No. 331,121, filed Dec. 16, 1981. This locking system comprises a spring-loaded ratchet on the operating ring biased into normal engagement with a toothed rack on another portion of the lens housing to lock the operating ring against axial displacement. The ratchet is manually retracted from the rack by depression of a spring-loaded button to permit movement of the operating ring to an alternative axial position. Accordingly, the button must be held continuously in the depressed position throughout the adjustment period. If it is desired to operate the zoom lens in a conventional unlocked mode, it is necessary to provide additional mechanical devices to lock the button in the depressed position. Such additional mechanical devices undesirably increase the overall cost and complexity of the zoom lens.
The present invention overcomes the problems and disadvantages of the prior art by providing an improved zoom lens having a relatively inexpensive and simple integrated locking system for movement between a locked position preventing axial adjustment of the operating ring to correspondingly lock the zoom lens in a selected magnification setting and an unlocked position permitting axial movement of the operating ring.